It is difficult to make direct enlargements from color transparencies on reversal paper, since the conditions with respect to producing the right adjustment of the color mix filters of the enlarger, depend on:
1. The color sensitivity of the photographic paper used in each case,
2. The departure of the colors of the transparency from neutral gray (color cast or tint),
3. And the personal taste of the photographer with respect to the way in which the colors are to be reproduced.
So far, there have two main ways of measuring the filtering effect and making the desired adjustment thereto:
1. Judging the color transparency or a test print by eye, and
2. Using an electronic color analyser.
Trouble-free operation of such an electronic color analyser is only possible with transparencies when all colors of the picture are balanced in respect of their areas, that is to say when no one color is dominant, because, otherwise, the dominant color would be oppositely filtered out and colors would be changed in an undesired way. Furthermore, an electronic color analyser does not make possible any direct testing or control of the color shades produced or desired by a given photographer.
It is known, that, on judging colors by eye, the color hue and density of an image on photographic paper may be exactly evaluated under normal lighting conditions. This, however, is not the case for an enlarged image in a dark room, because the eye of a person gets used to any basic coloring of the light and will see this light, within certain limits, as "white" light in all cases.
Producing the right color adjustment of the enlarger or printer is, in all cases, dependent upon absolutely evaluating or measuring the color properties of the light coming from the enlarger.